Freezing and thawing affects the properties of aggregates at or near the soil surface. Depending upon the amount of water, clay, and organic C present, these climatic processes may either decrease or increase aggregate stability. Recent research has suggested that a relatively small number of freeze-thaw (F-T) cycles may increase the stability of soil aggregates, when measured by wet sieving field-moist aggregates. The objectives of this laboratory experiment were to quantify the aggregate stability response of relatively wet aggregates from four different soils to up to 4 F-T cycles and, secondly, to identify a threshold number of F-T cycles for each soil below which aggregate stability increases but above which it decreases. The experiment was conducted as a split-split plot design, with six replications. Soils (4) were main plots, freeze-thaw cycles (either 0, 1, 2, or 4) were sub-plots, and sampling depths (0-15 and 15-30 mm) were sub-subplots. The Ap horizons of four soils were studied: a Palouse silt loam (Pachic Ultic Haploxeroll) from Pullman, WA, a Portneuf silt loam (Durixerollic Calciorthid) from Kimberly, ID, a Sharpsburg silty clay (Typic Argiudoll) from Lincoln, NE, and a Barnes loam (Udic Haploboroll) from Morris, MN. Each soil was at a matric potential of about -33 kPa, with water contents ranging from 0.22 to 0.27 kg/kg. Moist soil was packed by tapping to a dry bulk density of 1.15 Mg/m**3 into brass cylinders 50 mm high with inside diameters of 27.5 mm, then sealed in zip-lock, polyethylene bags to inhibit water loss. The cylinders were then placed into cavities in a polystyrene foam tray to insure that freezing occurred downward from the surface. The soil in the cylinders was then slowly frozen convectively without access to additional water at -5 degrees C for 48 h, then thawed at +6 degrees C for 48 h for each freeze-thaw cycle. The soil from both the 0-15 mm and 15-30 mm layers in each cylinder was sieved to obtain field-moist, 1- to 4-mm aggregates. These were vapor-wetted to 0.30 kg/kg, then wet sieved for three min to measure aggregate stability. Preliminary results revealed large differences in aggregate stability from one soil to another, due primarily to differences in clay and organic C contents. For all soils tested, the aggregate stability of field-moist aggregates increased from 0 to 4 F-T cycles. The largest increase in the 0-15 mm layer occurred with the first freeze-thaw cycle. The stability of the 0-15 mm sample exceeded that of the 15-30 mm sample for nearly all soils at all F-T cycles. These differences were minimal for 2 or more F-T cycles for all soils but the structurally weakest Portneuf.

Dr. Gary A. Lehrsch
USDA-Agricultural Research Service
Northwest Irrigation and Soils Research Laboratory
3793 N. 3600 E.
Kimberly, ID  83341-5076   (U.S.A.)
Phone: (208) 423-6508
Fax: (208) 423-6555
E-mail: Lehrsch@Kimberly.ars.pn.usbr.gov